1. Field of the Invention
The present invention relates to an underground exploration apparatus using an alternating current test method. More specifically, the present invention relates to an underground exploration apparatus for selectively probing a specific substance embedded in the ground or incorporated into the ground by: applying, between two different positions on the ground as a probing region, two alternating currents having different frequencies selected on the basis of the frequency characteristics of the dielectric constant of a substance to be probed; and measuring a potential difference between alternating voltages resulting from the currents applied to the ground at other two positions.
2. Related Background Art
A method of probing the ground involving the use of an alternating current test method in which an alternating current is caused to pass through the ground as a probing region has been conventionally known as a method of probing the ground. The method of probing the ground involving the use of an alternating current test method is disclosed in each of Japanese Patent Application Laid-Open No. H07-012766, Japanese Patent Application Laid-Open No. H09-127253, Japanese Patent Application Laid-Open No. H10-293181, Japanese Patent Application Laid-Open No. 2000-028743, Japanese Patent Application Laid-Open No. 2001-074850, and Japanese Patent Application Laid-Open No. 2002-156460.
The dielectric constant of soil-constituents or of a chemical substance in a soil depends on a frequency owing to physical properties such as interfacial polarization and dipolar polarization. Accordingly, in recent years, a method in which attention is paid to the frequency characteristics of the dielectric constant of a substance to be probed has been examined as an alternating current test method.
Hereinafter, the method in which attention is paid to the frequency characteristics of the dielectric constant of a substance to be probed will be described.
As shown in FIG. 2, soil-constituents and chemical substances in a soil have various dielectric constants. However, a trace amount of barium titanate having a dielectric constant of 1,000 in a soil having a dielectric constant of about 5 to 40 does not largely change the impedance of the soil, so the impedance is kept nearly constant. That is, a conventional electric probing method does not allow a trace amount of an embedded substance to be probed.
However, the dielectric constant is a physical parameter resulting from a polarization phenomenon occurring in a substance. Examples of the polarization phenomenon include factors such as interfacial polarization, dipolar polarization, ionic polarization, and electronic polarization. As shown in FIG. 3, the dielectric constant has frequency characteristics with which the dielectric constant largely changes at a frequency corresponding to each polarization factor. Of those polarization factors, the interfacial polarization and the dipolar polarization are each a phenomenon occurring in a low frequency region.
As shown in FIG. 4A, the interfacial polarization is a phenomenon caused by the movement of charged particles due to an electric field applied to a soil. Therefore, a polarization speed is largely affected by, for example, the mass and charge amount of the charged particles, the quality of the soil, and the viscosity of a medium. That is, the heavier the charged particles, the lower a polarization follow-up critical frequency. In contrast, the lighter the charged particles, the higher the polarization follow-up critical frequency. In addition, the polarization speed is less susceptible to a Coulomb force as the charge amount of the charged particles reduces, so the polarization follow-up critical frequency reduces. In contrast, the polarization speed is more susceptible to a Coulomb force as the charge amount of the charged particles increases, so the polarization follow-up critical frequency increases. When the soil is a porous substance or is composed of fine particles, the polarization follow-up critical frequency reduces because interfacial fluidity is extremely low. In contrast, when the soil is composed of large particles or fluidity is high at a particle interface, the polarization follow-up critical frequency increases. As described above, the polarization follow-up critical frequency corresponding to the interfacial polarization includes information on the soil and the charged particles.
As shown in FIG. 4B, the dipolar polarization is a phenomenon in which a molecule itself constituting a substance is polarized by reason of its molecular structure and the molecule is rotated by an external electric field to change its direction, that is, its orientation to cause the substance to polarize. Therefore, a polarization speed is largely affected by, for example, the size, moment, and shape of the molecule. That is, the larger the molecule, the lower a polarization follow-up critical frequency. The smaller the molecule, the higher the polarization follow-up critical frequency. In other words, the polarization follow-up critical frequency reduces when the molecular orientation is hardly uniformed by an external electric field. The polarization follow-up critical frequency increases when the molecular orientation is easily uniformed by the external electric field. As described above, the polarization follow-up critical frequency corresponding to the dipolar polarization includes information on a molecule of a substance in the soil.
The polarization follow-up critical frequency corresponding to each of the interfacial polarization and the dipolar polarization described above is a physical parameter to be determined depending on the quality of a soil and the physical properties of the substance and molecules in the soil. In particular, the polarization follow-up critical frequency corresponding to the dipolar polarization is a physical parameter inherent in a substance in the soil. Accordingly, selecting two frequencies sandwiching the polarization follow-up critical frequency corresponding to the interfacial polarization or the dipolar polarization allows one to selectively probe a substance to be probed.
As shown in FIG. 5A, when an alternating current is applied between two different positions on the ground as a probing region, two cases, that is, the case where a first frequency f1 is caused to pass through and the case where a second frequency f2 is caused to pass through are compared; provided that a polarization follow-up critical frequency inherent in a substance to be probed is sandwiched between the frequencies f1 and f2. The frequencies f1 and f2 may be different from each other only in a dielectric constant ε2 in a shaded region containing the substance to be probed. The equivalent circuit of FIG. 5A can be replaced with that shown in FIG. 5B. The dielectric constant ε2 in the shaded region is high at the frequency f1, so an alternating current easily flows. The dielectric constant ε2 reduces at the frequency f2, so an alternating current hardly flows. As a result, as shown in FIGS. 5C and 5D, the density of a current flowing in the shaded region is high at the frequency f1 and is low at the frequency f2.
FIG. 6A shows isoelectric lines at the frequency f1, while FIG. 6B shows isoelectric lines at the frequency f2. At the frequency f1, a current easily flows in the shaded region, so a potential difference is hardly generated between the ends of the shaded region. In contrast, at the frequency f2, a current hardly flows in the shaded region as compared to the case of the frequency f1, so a large potential difference is generated between the ends of the shaded region. FIG. 6C is a view obtained by: measuring ground surface potential distributions at the frequency f1 and the frequency f2; and determining a difference between the distributions. The potential difference map is equivalent to a ground surface potential distribution resulting from a fluctuation in current density caused by a difference in applied frequency. Therefore, when the shaded region containing the substance to be probed is localized to the probing region, the same ground surface potential distribution as that generated when a dipolar current source is present in the shaded region is obtained. In other words, the ground surface potential distribution in this case is a dipole pattern having a pair of a peak and a valley.
FIG. 6D is an electric field vector map obtained by treating the potential difference map for the surface of the ground shown in FIG. 6C obtained as described above. When graph drawing and a gray treatment are performed on the basis of the absolute value of an electric field vector, a portion directly above the shaded region having a large current density changed by a difference in applied frequency shows the largest absolute value and a high concentration.
As described above, a preceding application describes an apparatus which: performs electric probing by means of two frequencies sandwiching a polarization follow-up critical frequency inherent in a substance to be probed; measures ground surface potential distributions; and determines a difference between the distributions to allow selective ground probing of the substance. FIG. 7 shows the structure of an underground exploration apparatus in a preceding application. The ground probing system includes an oscillator, a voltage-current converter, an amplifier, a multiplier, and a low pass filter, and is structured such that it can perform probing at a single frequency at the same time. A retention unit holds measurements at respective frequencies, and a computing unit calculates a difference between surface potential distributions.
When the concentration of a substance to be probed in a soil is low or the total mass of the substance is small, a difference between a ground surface potential distribution by a first frequency and a ground surface potential distribution by a second frequency (the first and second frequencies sandwich a polarization follow-up critical frequency) is extremely small as compared to the absolute value of a ground surface potential. As a result, a change width may be several bits or less with respect to a conversion accuracy of 24 bits even when a high accuracy sigma delta-type AD converter is used for conversion. Cancellation resulting from a quantization error involved in the AD conversion makes it impossible to perform ground probing with high accuracy when the concentration of a substance to be probed is low or the volume of the substance is small.